A spool of this type is described in French Patent No. 2 147 473 and its corresponding United Kingdom Patent GB 1393607. It comprises only two parts made of moulded plastics material: an opaque one which is intended to support, alone, the tape during high-speed winding and unwinding thereof and the other, transparent, which is intended to be fixed in very supple manner in the first.
The opaque part is designed and produced to be very resistant. Firstly, the plastics material constituting it is reinforced with glass fibers. Secondly, its particular monolithic structure is, in principle, undeformable; in fact, this opaque part comprises a side element integral with a winding sleeve and a drive hub via a massive part. In this part are formed housings in which penetrate fingers integral with the transparent side element. The massive part then comprises very thick elements constituted by two rings joining an intermediate sleeve to the winding sleeve and to the drive hub, respectively. This massive part is very rigid and its rigidity is increased by ribs located between said housings and in particular joining the two sleeves together.
Consequently, the opaque part is very rigid and should be capable of supporting the efforts which stress it without deformation of the winding sleeve and without even partial transmission of these efforts to the drive hub which should therefore not be deformed.
In fact, the winding is slightly deformed when the magnetic tape is wound and this results in the magnetic tape pad not being geometrically perfect in that its side faces are not strictly plane and the tape floats laterally by a short measure during its advance, which disturbs the quality of the recording and the restitution of the signals. This also results in the disymmetrical contraction of the winding sleeve being accompanied by a tightening of the drive hub on the driving pin, rendering dismantling of the spool, not smooth and easy, but difficult due to the wedging.
These shortcomings occur even though the deformation of the winding sleeve is in fact very slight.
In order to avoid them, the step to be taken has always been the same: rigidify and reinforce the resistance of the central part of the spool. In this way, three-part spools made of plastics material employ a box structure in which the two lateral parts cooperate with an intermediate part.
This structure should, in principle, be satisfactory but, in practice, presents the same defect, even though it is attenuated.
In the two-part spool, the transparent side element comprises, on the one hand, projecting fingers penetrating in the housings in the opaque working part and, on the other hand, a central tubular sleeve of short height fitted around the drive hub. Free spaces are provided between the transparent fingers and the opaque sleeves as well as between the central transparent sleeve and the drive hub. Assembly of the two parts is obtained by depositing, before assembly, an adhesive which remains supple upon use, which replaces said spaces; consequently, the transparent part does not participate in the resistance of the opaque part.
On the contrary, in the three-part spool, assembly is effected by ultrasonic welding. Experience has shown that it is extremely difficult to effect such welding, as it is associated with the control of the clearance between the parts to be joined, which clearance is difficult to master due to the large number of identical moulds which are necessary to mass-produce the spools. Moreover, it is virtually impossible to render compatible any one of the moulds for manufacturing one part with any one of the moulds for manufacturing the second part and with any one of the moulds for manufacturing the third part, without having to make frequent adjustments or having to use specific assembly tools.
Under these conditions, either the clearance is too great and the relative positioning of the parts of the spool is defective, or the clearance is insufficient and the geometrical quality of the tape pad is doubtful, the transparent part not being able to vibrate freely.
It is an object of the present invention to design and produce a two-part spool so that all the difficulties of moulding and assembly are transferred to the opaque part and the resistance of the spool is ensured essentially by the opaque part virtually without the participation of the transparent part.
In this perspective, the present invention adopts a step contrary to the teaching of the prior art: it waives constituting a box structure between the winding sleeve and the drive hub, with or without the transparent part. Despite that, the invention aims at improving the quality of the tape pad of the two-part spool and at preventing the drive hub from contracting upon use, ensuring on the working part, between the winding sleeve and the drive hub, a supple connection thanks to which the sleeve may contract, but whilst conserving its cylindrical nature, whilst the hub must not deform at all, and concomitantly ensuring on the transparent part, between its inner projecting elements, a likewise supple connection thanks to which the above-mentioned mode of deformation of the working part is not disturbed but, on the contrary, corrected; it is not a question of a participation in the resistance but of a correction to the second degree of a possible lateral deformation of the supple connection of the opaque part. The purpose mentioned above is also attained by connecting the drive hub to the rest of the spool by means assimilable to a median semi-articulation and by placing between the two parts two ultrasonic welding beads under conditions such that the cylindrical nature of the tape pad is not affected and that the geometry of the spool as a whole is not disturbed.
Furthermore, the invention aims at the flatness of the side element of the opaque part being perfectly ensured and stabilized upon use in that the moulding of the central part of said opaque part and the winding of the magnetic tape must not influence nor disturb the flatness of said side element.